purifying tranny fluid

If one wanted to replace the ATF+3 in his AW-4 with the proper Dexron, how would you go about it to fully flush out all the old stuff?

I would at least do it by running it out the cooler lines while running, but is there such a thing as a tranny flush, like one can use on an engine?

How many times should it be pumped out and refilled to make sure there are no mixing problems?

Thanks in advance.